Traveling wave tube systems



July 2, 1963 E. c. Dr-:NCH

TRAVELING WAVE TUBE SYSTEMS 3 Sheets-Sheet 1 Filed Feb. 26, 1958 i 0000000000 I if.

VE'N TOE C'. @5A/CH freek/5r /N f' D14/Aaa @y cm July 2, 1963 E. c. DENCH 3,096,481

TRAVELING WAVE TUBE SYSTEMS Filed Feb. 26, 1958 5 Sheets-Sheet 2 VWG. 2

/0 a l I 3 b n n n n I C 'l 'l l d a i 1 1 July 2, 1963 E. c. DENCH 3,096,481

TRAVELING WAVE TUBE SYSTEMS Filed Feb. 2e, 1958 3 Sheets-Sheet 5 /N VEA/Toe 50u/,4R0 CD5-NCH United States Patent C) 3,096,431 TRAVELEJG WAVE TUBE SYSTEMS Edward C. Dench, Needham, Mass., assigner to Raytheon Company, Lexington, Mass., a corporation of Deiaware Filed Feb. 26, 1958, Ser. No. 717,782 Claims. (Cl. 325-17) This invention relates to a traveling wave electron discharge device of the backward wave type adapted to operate as an oscillator to transmit signals over a plurality of preselected frequencies in a radar or communications system and, more particularly, to a system for progressively tuning one or more of said backward wave devices `from one selected kfrequency to another for transmission of a single pulse or series of pulses of elect-rom-agnetic energy :at said preselected frequencies and simultaneously to provide a plurality of locking signals for rendering receiving means operative at `each frequency for the reception of said transmitted signals.

The desirability of transmitting and receiving signals over a plurality of individual frequencies in an effort to avoid loss of communication by substantial concentration of noise signals at a particular frequency is readily appreciated. However, the transmission of signals in the aforementioned manner over a plurality of preselected frequencies, lor even at random, generally requires an oscillator of extreme flexibility accompanied by accurate electromechanical tuning apparatus for peaking the oscillator to each selected frequency over a relatively wide frequency band. It is generally dithcult to obtain a wideband amplier of the high power type which is capable of rapid tuning to a succession of individual frequencies and at the same time actuating individual receiving channels which are tuned to the corresponding transmitting frequency.

A traveling wave tube of backward wave type is voltage tunable over a broad-band of frequencies, and oscillation can be initiated in this device by adjusting the beam current above a critical cut-oit value, arbitrarily designated `as I0. A backward wave tube of this type is described in detail in the copending application of Edward C. Dench and `Palmer P. Derby, entitled Electrical Systems, Serial No. 588,486, tiled May 31, 1956, now U.S. Letters Patent No. 3,038,067, issued June 5, 1962, and in the United States Patent No. 2,888,649 of Edward C. Dench et al., Serial No. 562,472, led January 31, 1956, the backward wave tube being described later in this application. In some applications, therefore, it is desirable to voltage tune va device or tube of this type over a wide band of frequencies to transmit a plurality of signals at selected frequencies, and at the same time to provide for the reception of said transmitted signals ata frequency corresponding to each transmitting frequency.

In accordance with the invention, therefore, an electrical system capable of transmitting and receiving electromagnetic energy over a plurality of selected frequencies can be achieved by providing a backward wave voltage tunable device which is fed by a plurality of injection dscillators generating a plurality of locking signals within the frequency range through which the device or tube is tunable, providing voltage regulating means to initiate oscillation at each of the desired locking `frequencies determined by the injection oscillators, and further providing `an intermediate yfrequency oscillator which beats with each injected signal to produce sideband signals above `and bel-ow each injection frequency. A lter is then provided lfor filtering out one of the sideband signals, while the remaining sideband signal is fed to a second mixer which beats with each incoming transmitted signal at a frequency corresponding to the frequency of the locking signal transmitted by the backward wave device; and

3,096,481 Patented `lilly 2, 1963 which, in this manne-r, provides for the reception of each transmitted signal over separate channels. Moreover, the system is insensitive to concentration of the noise at any particular frequency and a variety of frequencies over a wide band can be transmitted in any random or preselected program.

Other objects and features of this invention will be understood more clearly and fully from the Ifollowing detailed description of the invention with reference to the accompanying drawings wherein:

FIG. l is a schematic diagram of an injection locked travelling wave oscillator system according to the invention; Y Y

FIG. 2 is a detailed view of a portion of the anode assembly of a backward wave oscillator tube employing a transverse magnetic field;

FIG. 3 is a reduced section view taken along the line 3-3 of FIG. 2;

FIG. 4 is a circuit diagram illustrating a preferred manner of practicing the invention; and

FlG. 5 illustrates graphically the input and output waveforms of the regulating stages in the pulse control circuitry for the backward wave oscillator.

Referring now to FIG. 1 showing the block diagram of the backward wave locked oscillator system, an antenna 1d is provided to transmit recurring signal pulses, such as radar pulses, 4and the like, when connected by coaxial transmission line 12 to the end adjacent to the electron source of the signal transmission network 13 of backward wave tube 14. The backward wave tube oscillator 14, as shown, includes a grid 20, an acceleration electrode 22, and a cathode 15 positioned at the end of the signal transmission network 13 and provided with a heater, not shown. The purpose of the cathode 15 is to emit electrons, which, under the influence of the proper electrostatic and magnetic elds produced in the space adjacent the signal transmission network, will travel along paths adjacent to a series of interdigital fingers 16 forming said network and, after interaction with any signal present in the network, will impinge on collector electrode 17 of the signal transmission network 13, which serves las an anode. Signal transmission network 13 is maintained at the same potential as the collector electrode 17, or at some other potential relative to the cathode. The structural details of the cathode 15 and collector electrode 17 and the Iremaining elements and electrical connections comprising the backward wave tube 14 will be described below. Extending adjacent interdigital ngers 16 and forming a space through which the electron beam travels is an elongated electrode 18, commonly referred to as a sole, which in this embodiment is maintained negative with respect to the cathode 15 by a 700 volt power supply 19. In like manner, the grid or control electrode 20 is maintained negative with respect to the cathode by a 500 volt power source 21.

Voltage tuning of the backward Wave tube to the individual injection frequencies connected to the input of the backward wave tube is accomplished by effectively changing the Soleto-anode voltage by control of the cathode-to-.anode voltage by means of regulation circuit 25 in series with a 1500 volt anode supply 23 which, in turn, is connected to pulse circuitry in FIG. 4 in a manner to be described later. The sole 18 is maintained at .a constant voltage reference with respect to the cathode. In this manner, the anode voltage level selects the frequency at which the backward wave tube is tuned, independently of a trigger or accelerator pulse which is applied to the accelerator electrode to raise the beam current above the aforementioned I0 value in order to initiate oscillation in the backward wave tube at the selected frequency. It should be noted that the beam current can also be controlled by providing circuits which control the potential on the grid Ztl instead of accelerator electrode 22. Thus, in FIG. 1 a plurality of locking signals are amplified by an isolating traveling wave amplifier 2.9 and injected by means of coaxial input lines 27 and 158 into the backward wave oscillator 14 at individual frequencies corresponding to frequencies at which the tube delivers an output When oscillating. in the present embodiment of the invention, these locking signals are provided within the frequency band through which the backward wave device is tunable by continuous wave magnetrons 30, 31 and 32 which are adjusted or tuned by their individual mechanical tuning mechanism or rods 39a, 31a and 32a to the selected frequencies. These tuning rods are adjusted in a manner which changes the inductance of the cavity by changing the dimension of the cavity. While the continuous wave magnetrons shown herein provide approximately 200 Watts of driving power to the backward wave tube 14, it should lbe understood that any oscillator or magnetron adapted to oscillate at the desired injection frequencies, can be used. j

Referring again to FIG. 1, the output of the magnetron 30 is fed by Way of a SiO-ohm coaxial cable 33 into a coax-to-waveguide transition 34. The inner conductor of the coax is terminated in a matched termination 35, such as a Wheeler termination, comprising a tapered metallic support connected to the outer conductor of rectangular waveguide 36 which is of a dimension suflicient to propagate energy at the lowest frequency selected by the injection magnetrons. In like manner, magnetrons 31 and 32 are connected to their respective `50-ohm coax cables 33a and 33h which, in turn, are connected to waveguide transitions 35a and 35b. In order to provide three individual locking signals which are sufficiently isolated, ferrite isolators 37, 37a and 37b are provided which operate in connection with isolation amplifier 29 to prevent undesired radiation of signals through the backward wave tube 14 during the non-transmitting cycle, to be described in detail later. These isolators comprise a rectangular section of waveguide in which is positioned ferrites 3S, 38a and 38h extending longitudinally adjacent the inner Wall of each of the three waveguide sections and which are impressed between opposite pole pieces of a permanent magnet 39, 39a and 39b to apply va transverse magnetic field in the region of the ferrite element. Any S-band isolator providing 20` to 25 decibel isolation with? an insertion loss of approximately 1.2, decibels can be used tto provide the required intersignal isolation. The three individual oscillator signals are then fed into a common coaxial waveguide 40 by way of three matching transitions 41, 41a and 41h which are connected to the input coupling coil 159 of the isolating traveling wave tube 29. In this manner, the three separate injection signals are amplied and simultaneously fed to the backward wave tube 14 to initiate oscillation of the tube at each of these locking frequencies as determined by the voltage levels establishing by the regulator circuitry 25 and pulse circuitry of FIG. 4 in a manner to be described in detail below. The injection oscillators, as noted, are run continuously and each is tuned by means of its tuning mechanism to the individual locking frequency established by the regulation circuit in order, progressively, to lock the backward Wave tube 14 to each injected frequency. Also, in .accordance with the invention, the individual injection .signals are fed by way of a standard coaxial connector 42 into directional coupler 43 which, fas shown, comprises a short arm coaxial lead 43a terminated in graphite termination support 43h and coupled by Way of aperture 43e to the inner conductor of the main waveguide 40.

A local oscillator 53 is provided to beat with each injection oscillator output by means of a broadband mixer section 54. The local oscillator 53` is tuned to 60 megacycles and is capacitively coupled to the center conductor 55 of coaxial mixer section 54 by means of an adjustable capacitive coupling device 56 having a threaded outer conductor, not shown, which may be rotated to vary the coupling distance of the inner conductor 57 of the local oscillator coaxial line 58 from the center conductor of the coax line 55. In this manner, the local oscillator frequency is mixed with the injection oscillator frequency by means of a mixer crystal 59, the nonlinearity of which causes the input signal frequency plus and minus the intermediate frequencyto appear in output coaxial line 6G as well as the intermediate frequency and input signal frequencies. These frequencies are propagated by way of the coaxial line 60 and another coax-towaveguide Wheeler transition into the rectangular waveguide 61 of a three-channel filter section 44. Waveguide 61 is provided with three apertures 62, 62a and 6217 which provide `a coupling to three tunable S-band cavities 63, `63m and y63b, each tuned to a frequency equal to the individual Ysigna-l frequency plus the IF frequency by means of micrometer adjusting screws 64, 64a and 64b. The S-.band cavities are provided with output iris coupling apertures 65, 65a and 65h which feed the energy into another rectangular waveguide section 66. By this arrangement, the tunable cavities act as filters to pass only the signal frequency plus the IF frequency for direct connection to a second mixer 67. This mixer is similar to crystal mixer 55, and mixes the filtered output signals with the return lsignals from receiving antenna 68 and coax 69 to form an intermediate frequency signal output by means of the second mixer crystal 70 prior to introduction of the mixed signals into an AIF amplifier of a conventional receiver, not shown. The second mixer 67 is further provided with an adjustable capacitive coupling '71 for `adjusting the mixer coupling to obtain an adequate signal mix in the well-known manner prior to feeding the intermediate frequency output signal to the conventional IF amplifier.

The rectangular waveguide structures 61 and 66 are each provided with graphite `terminations 74 and 75, respectively, which absorb reflected energy in the line in order to prevent standing waves which might occur due to only a part of the microwave energy passing through the resonant cavities 63, 63a and 63h. These cavities are loosely coupled t-o the rectangular wave guide 66 in order to provide a high enough Q to insure discrimination against the signal frequency minus the IF frequency from entering rectangular waveguide 66 which acts as an output feeder line for the filter 44. It should -be understood that any suitable coaxial coupling arrangement which is capacitively coupled from the main coaxial output arm 4t? to the side arm 43a portion of the coax 60 can be used to sample a portion of the oscillator signals being fed into isolation amplifier 2,9 and the backward wave oscillator tube 14. The'wdirectional coupler 43 samples a portion of the energy in coax 40 for the above-described mixing purposes.

In this manner, the backward wave `oscillator 14 is fed with three individual injection frequencies which are adjusted by means of the mechanical tuning slug on each magnetron to lock or pull the transmitting frequency of the backward wave tube 14 to that of the injection oscillators. 'Ihe injection filter cavities 63, 63a and 63h are preferably tuned -to pass signals occurring 60 megacycles above the signal frequency prior to mixing these signals with the incoming radar 4or communication signals from receiving antenna 68. As noted, the output of crystal mixer 7i) is coupled in known fashion to an intermediate frequency amplifier, no shown, the output of which is detected in a detector, not shown, and Ithen applied to a suitable radar indicator or display device, such as, for example, an A-scope described on page 524 of Radar Systems Engineering by Ridenour (Radlab Series, Volume 1). In `other embodiments, it should be Vunderstood that oscillators other than mag.- netrons may be used to provide locking signals for the backward Wave tube 14. Also, isolation and amplifier means can be used other than the traveling wave tube 29, the operation of which will be described in detail 1n connection With the description of FIG. 4.

Referring now to FIGS. 2 and 3, the construction details shown therein do not form part of the invention and are not described in detail. They are, however, shown `and described in detail in the aforementioned copending application for Electrical Systems, Serial No. 562,472 of Edward C. Dench and Albert D. La Rue, led January 3l, 1956, now matured into United States Patent No. 2,888,649, issued May 26, 1959. In FIGS. 2 and 3, a backward wave tube 14 is shown which comprises an anode assembly 81 which includes the energy propagating structure or signal transmission line including the elongated electrode `or sole 18 which, as noted, is maintained negative with respect to the interdigital lingers '1t formingauodeV delay line 13, a'lead-in assembly 82 and an output coupling means 83. In addtion, there is shown an electron gun mounting assembly 84 including the cathode 15 containing a heater, not shown, the control grid 20, an input coupling means 85 for the coaxial transmission line 27 of FIG. l and -a transverse magnetic held-producing means Sei-87, a portion of which is indicated in FIG. 2.

The interdigital lingers 16 comprising the signal transmission line include a plurality of members which extend from oppositely-disposed annular members 88, $8', respectively. These members are secured by screws, not shown, -to the shoulder portion of a cylindrical thermallyconductive ring 89, 89 to which is hermetically sealed a pair of oppositely-disposed cover plates 90 and 91.

The sole 1&5` consists of a cylindrical block of material, such as copper, having a centrally-located aperture 93 to permit connection of lead-in assembly 82 and to allow for passage of external circuit-connecting leads.

Referring more particularly to FIG. 2, the lead-in assembly 32 comprises an electrically-conductive cylindrical sleeve 94, which is inserted in an aperture in cover plate 90. Iuterconnecting metal sleeve 94 and outer metal sleeve 95 is a section of cylindrical glass tubing 96. The other end of sleeve 95 is provided with a glass seal 97 for sealing the tube 14 after evacuation. The assembly 82 is arranged perpendicularly to cover plate 90 of tube 14 and further includes an elongated electricallyconductive tubular supporting cylinder 98, which serves as a main support for sole 18 and is affixed at one end to the periphery of aperture 93 in sole 1S. The outward end of cylinder 98 contains an outwardly llared portion 99, which is connected to the inner surface of outer metal sleeve 95. The necessary leads for the electron gun are fed through supporting cylinder 5S and are insulatedly supported therefrom by one or more glass beads 100. The interdigital fingers comprising the signal transmission line 13` lare arranged concentrically with sole 18 and are separated from the circumferential wall `101 of the sole to form an interaction space 102 through which the stream of electrons generated in the tube passes. The coaxial output coupling means 83 is sealed in an opening of wall 39 of .the anode and is impedancematched to the interdigital delay line 13. The inner conductor 103 of coaxial output coupling means 83 is connected to a linger at or adjacent the end of the periodic anode delay line 13 `adjacent to the electron gun.

The backward wave tube 14 is provided with a collector electrode 17, as shown in FIG. 3, for intercepting electrons after one traversal of the arcuate interaction space. This collector electrode may take the form of a projection from the back wall 89 of the interdigital delay line 1.3. In some instances, however, the collector electrode may be omitted and the electron stream made reentrant. Furthermore, the sole 18 may be either primarily or secondarily electron-emissive Electron gun assembly S4 for the backward wave tube, shown in FIGS. 2 and 3, as noted, includes the grid 20, the cathode with a heater inserted therein, not shown,

and an acceleration electrode 22, as shown in FIG. 2. More particularly, the cathode 15 is shown, by way of example, as a rectangular body provided with a circu-I lar bore, not shown, in which the heater is inserted. The cathode body 15 has at least the surface facing the accelerating anode 22 coated with an electron-emissive material, such as a compound of barium. Cathode 15 is positioned within the wall 101 of sole 18. rIhe cathode -lead 106 is connected electrically .to the cathode 15. Gne end of the heater, not shown, is connected to the inner wall of the cathode body, while .the other end of the heater is attached .to the heater lead 107, shown in FIG. 2.

The auxiliary electrode 22 which, in effect, is an accelerating `anode serving to aid in the production of the desired electron beam trajectory, is insulatedly supported Vtrom ange portion 92 of sole 18. The auxiliary electrode lead 108 is attached to the auxiliary or acceleration electrode 22.

A suitable electric field between anode and sole may be obtained by means of a voltage applied therebetween. The sole 1S may be negatively biased with respect to the cathode by means of the supply source 19 of voltage connected between the cathode lead 106 and tubular sleeve 98, by way of metal sleeve 95. The grid 20 may be maintained at negative potential with respect to the cathode by `grid supply source 21 of voltage connected between cathode lead 106` and grid lead 10S, only partially shown in FIG. 2. Similarly, the signal ytransmission netwo'rk or anode delay line 1'3, as shown in FIG. 3, is maintained at a positive potential relative to the sole and cathode by means of anode supply source 23 of voltage connected between metal sleeve 94 and regulation circuit 25, which is connected in turn to the anode transmission line 13 and cathode lead 106. As noted, the auxiliary or :acceleration electrode 22 is pulsed at a positive potential relative to the cathode by means of supply source 24 of voltage connected in series with the pulse control circuitry of FIG. 4 between leads A and B which are connected to corresponding leads A and B of FIG. 4.

A uniform magnetic -tield transverse to the direction of propagation rof the electron beam is provided either by a penmanent magnet `or an electromagnet having cylindrical pole pieces S6 and S7 nadially positioned on or adjacent the tube. Pole piece 86 is apertured to receive the lead-in assembly 82 and pole piece 87 `is apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 102 between sole 1S and cylindrical transmission network 13. By proper adjustment `of the magnitude and polarity of the magnetic and electric iields, the electron beam may be made to follow a circular path about interaction space 102 under the combined influence of these transversely disposed ields.

As noted, the radio frequency energy generated in the interaction space 102 traveling along signal transmission line 13 sets up la high frequency electromagnetic ield which may be analyzed as a series of space harmonics, some of which travel in `one direction (clockwise) along the anode structure, the `others of which :travel eounterclockwise, and all of which travel with differing phase velocities. If the electron beam is synchronized with the proper space harmonic, interaction of the beam and the space harmonic will result in the production of oscillations w-ithin the tube. The oscillations can be controlled by changing the electron beam above or below the critical value I0. The energy travels through the aforementioned space toward the electron `gun and is extracted at the gun end through the signal transmission line 13 by way of the coaxial 'output line 33. Backward wave tube 14 further -includes the input coupling assembly 85 comprising an inner conductor 109 and an outer conductor 110 ycoaxially arranged with respect to one another. The inner conductor 109, as shown in FIG. 3, is connected to one of the -ngers 16 at or adjacent to the anode transmission'line 13 electrically removed tromthe electrongun while the outer conductor 11) may be attached to the cylindrical wall 89 ofl the anode assembly S1. The input coupling means y85, as well as the output coupling means 33 need not be coaxial. F or example, the energy may be coupled to or from by means of a waveguide.

It lshould be understood lthat the delay line or signal transmission network 113 may not be of the interdigital type, but may be any suitable periodic delay structure such Ias a helix, disc-loaded waveguide, or the like. Also the backward wave oscillator may be of .the well-known linear type not requiring a magnetic field transverse to the llow of electrons in the interaction space. As noted, tuning of the backward wave oscillator may be accomplished by varying the voltage between the signal titansmission line 13 and sole 18, as will be described in detail below. However, tuning of the backward wave tube 1d, also may be accomplished by varying the magnetic iield strength, either by varying the position of the magnet pole pieces, in the case of a permanent magnet, or by varying the electric current in vthe case of an electromagnet having ya coil `surrounding the core. Variation of both the electric field and the magnetic field simultaneously, of course, is possible.

Referring now to FIG. 4, there is shown a circuit diagram of a preferred embodiment of the voltage regulator system described generally in connection with FIGA l. In FIG. 4, wherethe elements are shown in FlG. l, the same reference numbers are used. 1n FIG. 4, the control pulses are fed to the acceleration electrode of the backward wave tube to set the tube into oscillation at the three voltage levels corresponding to the three individual transmitting frequencies. The frequency at which the backward wave tube is operated is determined by the pulse timing and sequence circuitry in the following manner: System trigger multivibrator 1.29' is a conventional bistable tree-running multivibrator comprising an input tube `12,1 and an output tube 122, the cycle of operation being determined according to well-known multivibrator operation and, in particular, :according to the value of the 500` micromicrofarad timing capacitors 12?#` and 124i. The output waveform of the system trigger multivibrator 120, shown in FIG. a, is differentiated by capacitor 12S and plate resistor 126 in combination with the input impedance ot a `one-quarter microsecond delay network 127. The delayed `differentiated output of multivibrator 12.11, as shown in FG. 5c is amplied by inverter triode '119 and is used to trigger the acceleration pulse width multivibrator 128, the output of which, as shown in FlG. 5b, is amplified and inverted by pulse amplifier 129- to drive the backward wave tube 14 int-o yoscillation after a value ot sweep voltage has been applied to tune the backward wave .tube to one of the three output frequencies in a manner which will be described in detail later. Also to be described is the bistable multivibrator 1311 which, in connection with monostable multivibrator 132, generates lan adjustable widthgate to set the particular voltage level and thereby, to Iselect one of the three individual frequencies to be transmitted by the backward wave tube 14 at the times determined by the sequential clamping circuit comprising clamping multivibrators 134i and 1135 which determine the lirst of the three rtnansmi-tting frequencies, multivibnators 136 and 137 which determine the second of the transmitting frequencies, and'multivibrators 138 and 139 which determine the third of the transmitting frequencies.

Referring again to system trigger multivibrator 1211, the output gate at plate 141i is also fed by way of conductor 121B to an inverter stage 142, a timing multivibrator 1414 and ia power amplifier tube 146 having a ten kilovolt isolation transformer 14d connected in its plate circuit. ln order to permit sufficient time for llocking pulse amplifier 29 of FlG. 1 to reach its maximuum output before the backward wave tube 14 is pulsed, a timing pulse output of timing multivibrator .144 is amplified by power amplier tube '1% `and fed by way of the transformer secondary leads, designated E and F, into the corresponding leads of the control circuitry for the locking pulse traveling wave tube 29 of FlG. l at approximately onehalf microsecond earlier than the time at which the timing pulse from trigger multivibrator 1211 is fed to the remainder of the system. Locking amplifier on time multivibrator 144 determines the length of time during which the traveling wave amplifier 29 is supplying locking power to backward wave tube 14. In operation, the isolating traveling wave tube 29 is held at a negative potential by a -source of negative voltage 15b of approximately minus 8 kilovolts applied to its beam `forming cathode 151, while the acceleration anode 152, in the absence of an acceleration pulse, is biased to cut-ofi with respect to the cathode 1:11 by a bias supply 153 of 750 volts direct current in series with the secondary of isolation transformer 148 of FlG. 4, by way of leads designated E and F. The traveling wave tube 29 is a high power wide band pulsed amplifier having a helix Yand a beam collector 156 adjacent output coupling coil -157 to which is connected input coaxial line 27 of the backwardV wave tube 14 by way of coaxial line 158. At the voltages applied to traveling wave tube 29, a gain of approximately 20 decibels is realized from asignal fed to input inductive coupling 159 from the directional coupler 413 which is ample to drive the backward wave tube 14 and at the same time prevent transmission or feed-through of signals from the injection magnetrons 30, 31 and 32 during the nontransmitting portion of the system operating cycle. lt should be understood that while any high powered traveling wave amplifier tube may be employed as a pulse isolating means, the present traveling wave tube is similar to that described in an article entitled The Design of High Power Traveling Wave Tubes, by M. Chodorow and E. I. Nolos on pages 649 to 659 of the Proceedings or" the LRE. for May 1956. in this manner, the backward wave oscillator 1d and the trigger multivibrator system is effectively delayed by the small fixed value of approximately one-quarter microsecond to allow sufficient time for the traveling wave tube Z9 to reach its maximum output before the backward wave tube is pulsed. Thus, the traveling wave pulse is approximately one-half microsecond wider than the pulse transmitted by the backward wave tube shown in FIG. 5b.

Referring again to the one-quarter microsecond delay network 1127, the ditlerentiated output of multivibrator 12) is introduced into the delay network and traverses the delay line inductances 171i 'and 1711. The delay network is terminated in its characteristic impedance by output resistor 172 and grid resistor 173 which is returned to the alternating cururent ground through a filter capacitor in the 250 volt power supply, not shown, connected to terminal B1. It should be understood that since delay network 1127 is terminated in its characteristic impedance, reflections `along the delay line do not occur and an accurate delay of one-quarter microsecond `occurs in the pulse which is fed to grid 174 of buffer tube `175. The latter grid is biased in a manner to amplify only the negative product of the differentiated time impulses, as shown in FIG. 5o. The amplified and inverted negative trigger is applied by way of coupling capacitor 176 to grid 177 of the pulse width multivibrator i128 to initiate the pulse waveform which is shown in FIG. 5b. This input grid is biased to cutoff by means of a 50,000 ohm resistor 178' and negative bias source 179 `of approximately 10 volts. Resistor 178` in combination with resistor 179 maintains a constant negative voltage of l0 volts on the grids of the multivibrator 128. Prior to the entrance of a trigger pulse into the multivibrator 128, the grid 1811 of the output halt of the multivibrator is held at a small positive Voltage by way of timing resistor 182 which maintains conduction in that half yof the tube. When a trigger is introduced into the grid 177, conduction in this half of the tube occurs, which drives the grid 181 to cutoff thereby raising the bias level set on grid 177 to accelerate the switching action toward conduction. Capacitor 183 discharges through timing resistor 182 and plate load resistor 184 of the first half of the multivibrator 128. This process continues until the grid 181 reaches a potential which is suiiicient to again initiate conduction in that half of the tube and maintain its grid at `a slightly positive potential with respect to its cathode. When an output pulse is thus formed, by the return of the output half of the tube to conduction, the duration of the output pulse is determined particularly by reseistor 182 and capacitor 183 and in part by the plate and load resistances of that half or" the multivibrator. This square wave output is then fed to grid 185 of a pulse amplifier 186 which was biased to cutoff by a 75 volt negative source 187 and a 1500 ohm grid resistor 188.

Pulse amplifier tube 186 is a type 3E29 tube having a screen 189 which is maintained at a positive potential of approxima ely 500 Volts by the 500 volt bias source 190 by way of a 1500 ohm screen resistor 191. The plate 192 of tube 18d is connected to the primary winding 193 of a pulse transformer 1124 and to a positive potential of 3000 volts from a power supply, not shown, connected at B-l-Z. The pulse transformer 134 is provided with 10 kilovolts isolation from primary to secondary. When tube 186 is driven into conduction by positive square waves applied to its grid 185, the secondary 195 of the transformer 134 applies a positive square wave to the `accelerator plate 22 of the backward wave tube 14 through conductors designated A and E, which are connected in series with the negative 200 volt bias supply 24 shown in FIG. 1. ln this manner, the accelerator of the backward wave tube is pulsed and oscillation occurs at the particular frequency to which the backward wave tube is tuned by means of the sweep and timing circuitry which will be described in detail. It should be understood that prior to initiation of oscillation in the lbackward wave tube 14 by pulsing its accelerator through conductors A and B, the traveling wave amplifier has been readied to transmit its full output by the locking pulse which is applied to terminals E and F at a time approximately onequarter microsecond earlier than the accelerator pulse.

It should also be understood that triode inverter and isolation amplifier 142 and the traveling wave tube timing multivibrator 144- are similar to buffer tube i175 and multivibrator 128, respectively. Also, power amplifier tube 186 and amplifier tube 146 provide similar outputs separated by one-quarter of a microsecond. While other circuit arrangements can be provided for generating a pulse to turn on the traveling wave tube, the present symmetrical circuitry is a simple way to achieve this accurate timing.

The circuitry `for sweeping the backward wave tube over a broad band for frequencies and of clamping the backward wave tube to one of the three voltage levels corresponding to one of the three selected frequencies prior to the arrival of an accelerator pulse will now be described. 1n other words, the backward wave tube is tuned to an individual frequency as determined by a preset voltage level applied to its sole 18 before the accelerator pulse initiates oscillation at the preselected frequency. The backward wave tube is swept through the predetermined band of frequencies by means of a sweep thyratron 200 and sweep capacitors 201 and 202 which generate a sawtooth sweep voltage. The change in voltage is applied by way of conductors C and D to voltage amplifier tube 204 and regulator tube 205, shown in FIG. 1, in the anode circuit of backward wave tube 14. The trailing edge of the output waveform of multivibrator 128 is used to fire the sweep thyratron 200 to discharge capacitors 201 and 202 to initiate the sweep excursion and to control the voltage regulation circuitry in the anode and sole of the backward Wave tube. This sawtooth is clamped at a preset level by a negative gate, as shown in FIG. 5g, `applied to anode 210 of clamping diode 211 by way of an :output conductor 212 from the multivibrator timing circuitry.

The negative square wave output from multivibrator 128 is fed to differentiating condenser 215 and resistor 216, the differentiated trailing edge of which appears as a positive output fed to the primary winding 217 of pulse transformer 218. This isolation transformer is provided with 10,000 volt breakdown insulation from the primary to secondary 219, which is connected to the grid 220 of tube 200 by way of input condenser 221. The grid 220 is normally biased to cutoff by an adjustable bias source 222 and input resistor 223. The plate 224 of tube 200 is connected in series with clamping diode 211 and by way of isolation resistor 225 to a thyratron supply source, not shown, connected at terminal B-l-3.

In operation, the positive voltage is fed to the thyratron 200 through the diode 210 and resistor 225V to charge the sweep condensers 201 and 202 to the level which supplies the voltage required t0 establish oscillation at the `desired transmitting frequency. Adjustable condenser 202 presets the grid level on ampliiier tube 204. When the positive transformer pulse arrives at `the grid 220, the thyratron 200 discharges the capacitors 201 and 202 to initiate the sweep excursion which is fed to the grid 230 of amplifier tube 204 by way of lines C and D. This tube is normally biased to the initial quiescent level of 4000 volts by a bias source 231 and by a 10 megohm grid resistor 232. A voltage drop across the 2G53 type tube 204 maintains the grid 233 of tube 205 at a bias level which sets the voltage applied to the anode 17 from the anode supply 23 to the desired initial voltage level of 4000 volts. This is accomplished by changing the conductivity of the series pass or regulator tube 205 in the usual manner. Thus, the positive sawtooth output from condenser 202 drives the grid 230 of the control amplier tube 204 in a manner Which applies a negative Sawtooth to grid 233 of the regulator tube 205. This produces a negative excursion of the voltage applied to the anode 17 of the backward wave tube 14 from 4000 volts down to approximately 3500 volts, thereby changing the frequency of the backward tube to the desired transmitting frequency, as shown in FIG. 5j.

It should be noted that the amplifier tube 204 and control tube 205 apply this sawtooth voltage to the anode of the backward wave tube between trigger pulses. In addition, this sawtooth is clamped to a preset level by a negative gate applied to the anode 210 of clamping diode 211, which level is determined by the multivibrator circuitry feeding conductor 212. The clamping action takes place as follows: When the thyratron 200 is fired by the differentiated pulse applied to transformer 218, a negative trigger pulse simultaneously is fed by way of conductor 240 to the input grids of bistable multivibrators 130, 134, 136 and 138. The input sections of multivibrators 134, 136 and 138 are normally nonconducting, whereas the input section of multivibrator 130 is normally conducting. This initial combination of conditions makes it possible to step a trigger successively through the multivibrator stages.

1n particular, the negative trigger fed to the input grid 234 of multivibrator 130 is taken from the output plate of multivibrator 128 and fed by way of a conductor 235 to differentiation condenser 236, which, in connection with the primary inductance 237 of transformer 238, forms a differentiation circuit which receives the differentiated pulse across the primary. Transformer 238 is a 10,000 volt isolation transformer having a secondary 239 which is shunted by a diode 241 poled in a manner to eliminate the positive portion of the differentiated pulse, and thereby, supply a negative differentiated pulse as shown in FIG. 5d to the grid 234 of bistable multivibrator 130 by Way of a conductor 242 to input condenser 243. A transformer pulse is also fed by way of conductor 240 to the grid of bistable multivibrator 134 through input condenser 245, to multivibrator 136 through input condenser 246, and to multivibrator 13S` through input condenser 247. The direct current level at the grid 234 of multivibrator 130 is set by means of timing resistor 250, a 100,000 ohm input resistor 251 and a 200,000 ohm resistor 252 which form a voltage divider network between the plate 253 of the output side of multivibrator 130 and the common cathode pulse lead 254. The additional resistance introduced into the circuit by means of common cathode resistor 252 raises v grid voltage on grid 234` rendering that section of the multivibrator conductive. Grid 255 is biased to cutoff by grid resistor 256 and, therefore, the input half of multivibrator 130 is normally conducting and the output section of the multivibrator is normally cut olf. As noted, the remaining input stages of multivibrators 134i, 136 and 138 are biased to a normally cutoff condition and are not aiected by the negative transformer trigger pulse applied to multivibrator 130. When this trigger is introduced into the grid 234, the voltage applied to input plate 257 rises and by multivibrator action causes the output half of multivibrator 130 to conduct and, thereby, lower the voltage applied to plate 253 as shown in FIG. e. This falling voltage is coupled by `a condenser 25S to a grid 259 of monostable multivibrator 1132. The input plate y261 of multivibrator 132 is driven in a positive direction by the negative input pulse applied to the grid 259 and rises for a time interval determined by the time constant of variable resistor 262 and variable condenser 263, which may be adjusted to set the length of the operating cycle. The positive square wave output produced at the plate 2611 of multivibrator 132 is fed by Way of conductor 265 and input condenser o to a grid 267 in the youtput section of multivibrator 134. This positive square wave or gating pulse is idifferenti-ated at the grid 267 of multivibrator 134 and only the trailing edge of this gate is effective since grid 267 is normally conducting. Mutlivibrator action lowers the voltage on the input plate 268, thereby feeding a negative pulse at the proper time interval through isolation diode 270 to clamping diode 211 to clamp the voltage at the first desired frequency level. However, during the time the rising voltage is applied to plate 261 of timing multivibrator 132, the sweep voltage is building up across the thyratron tube 2010 as shown in FIG. 5i. The end of this time interval, as shown at point p of FIG. 5i is determined by variable resistor 2621 and variable condenser 263. At this time the multivibrator 132, FIG. 5f reverts to its initial stable state, thereby interrupting the charging of the sweep condensers 201 and 202 at the first voltage level and triggering the grid 267 of bistable multivibrator 134.

As noted, the voltage applied to plate 268 falls due to multivibrator action, and the rise in voltage at plate 264 is shown in EFIG. 5h. This falling voltage at plate 268 is used to clamp the voltage built upon sweep condensers 201 and 202I at this particular level for the remainder of the interpulse interval. In other words, the negative pulse cuts iol the charging diode 21.1 by lowering the anode voltage and thereby holds the sawtooth voltage to the preset frequency level. This clamping pulse is shown in FIG. 5g and the gating pulse at the plate 261 of multivibrator 132 is shown in FIG. 5f. The width of this gating pulse determines the charging time of the thyratron plate 224, shown in FIG. 5l'. Thus, when the normally conducting `output half of bistable multivibrator 1134 is turned lofi, the negative gate, as shown in FIG. 5g cuts oi the charging diode 2111 in the aforementioned manner.

It should be understood that only the loutput half of one of the stages of multivibrators 134i, 136 and 138 conduct at one time to produce an output gate. This is because a normally closed push-to-reset sweep switch 275 initially places a 200,000 ohm resistor `252 in the grid circuit to bias the grid 234 of only the multivibrator 130 to an initially conductive state. This initial state could be achieved in connection with the power switch. The

12 input stages of multivibrators 134, 136 and 13S, as noted, are normally nonconducting and no output at isolation diodes 277 or 278 occurs when the negative pulse arrives simultaneously at the grids of the associated multivibrators by way of conductor 240. Y

in operation, therefore, the input section of multivibrator is initially conducting and is used to initiate activity in the widthgate timing multivibrator 132. A negative pulse applied to the input of multivibrator 132 generates a square wave Itiming pulse by way of ready line 265 to bias the grid 267 of multivibrator 134 to cutoff and by multivibrator action generate the negative clamping pulse shown in FJG. 5g, and sets the voltage level for charging capacitors 201 and 202. rl'his voltage level is maintained in order to determine the frequency at which the backward wave tube will be activated by an accelerator pulse. During the time this level is established, a second accelerator pulse from pulse amplifier 136 arrives at 'the acceleration anode 22 of the backward wave tube 14- and oscillation occurs at a frequency corresponding to the voltage level set on charging condensers 201 and 202. After the output section of bistable multivibrator 130 has been rendered conducting, it remains in that condition until it is reset at the end of the entire transmitting pulse sequence, as shown in FIG. 5e, by negative pulses from ythe plate 200 of the iinal stage of the conducting section of multivibrator 139 by way of reset line 231 to the grid 255. After monostable multivibrator 132 has fed a ready pulse to grid 267 of multivibrator 134, it returns to its first initial state at the end of the time interval set by resistor 262 and condenser 253. Multivibrators 4131i and 132 are thus inactivated during the remainder of the transmitting sequence. The trailing edge of the output of transformer 213 due to the triggering action of acceleration pulse multivibrator 12S, as shown in FTG. 5d, fires the thyratron 200 which initiates a new sweep applied to the grid 230 of regulator tube 204, shown in FlG. l. As the plate 210 of charging diode 211 is released, multivibrator is triggered by the input pulse to the grid 239 via conductor 291. The multivibrator 135 starts its RC timing action in the same manner as multivibrator 132 and at the completion of ithe time determined by variable resistor 2512 and variable condenser 293, as shown in FlG. 5j, multivibrator 136 is triggered by way of ready line 294 feeding a pulse to the grid 295 of multivibrator 136. The voltage on the plate 296 rises, as shown in FIG. 5l, which causes a corresponding voltage drop in plate 297, as shown in FiG. 5k. This voltage drop is applied by way of isolation diode 276 to the plate 210 of clamping diode 211, thereby arresting the excursion of the sweep voltage at point Q of FIG. 5i, across capacitors 201 and 202 at a new level awaiting the action of a succeeding accelerator pulse to start oscillation of the backward wave tube 1li at the second frequency corresponding to the new voltage level. At the trailing edge of the accelerator pulse a negative ,trigger is applied by way of capacitor 246 to reset multivibrator 136, thereby triggering multivibrator 137 to initiate its timing action. The time of this pulse is shown in PEG. 5m. ln this manner, the grid 299 of multivibrator 137 is triggered and starts its timing action in the same manner as multivibrator 135. At the completion of the RC time, multivibrator 138 is triggered via conductor 300 `and this multivibrator now holdsoif diode 211 arresting the excursion of the sweep at the lthird level as shown at point R. The third accelerator pulse triggers multivibrator 138 back to its original state by way of capacitor 247 and simultaneously triggers multivibrator 139 which feeds a reset pulse at the end of its timing cycle by way of line 281 to the grid of the iirst bistable multivibrator 1130, thereby `starting another pulse cycle. This output reset pulse occurs at approximately the same time as the discharge pulse iires thyratron 200 to initiate the start of another sweep.

It should be understood that any number of stages and any number of voltage levels may be established by providing additional timing circuitry as determined by the variable resistors and condensers in the multivibrator timing circuits in order to produce a plurality of output frequencies at which the backward wave tube is progressively tuned. It should also be understood that any number of magnetrons or other microwave sources may be used as drivers for the backward wave tube, which is not to be limited to operation in any particular frequency band.

This completes the description of the embodiment of the invention illustrated herein. However, many modifications and advantages thereof will be apparent to persons skilled in the art without departing from the spirit `and scope of this invention. Accordingly, it is desired that this invention not be limited tot the particular details "of theV embodiment'discl'osed Yherein except as rdefinedbyV the appended claims.

What is claimed is:

l. A radar system for alternately transmitting electromagnetic energy to a refiecting surface and receiving electromagnetic energy from said surface comprising a backward wave voltage tunable device having an oscillatory mode and an amplifying mode of operation and tunable through a predetermined frequency range, means for cyclically tuning said device through said frequency range, injection oscillator means for injecting a plurality of locking signals into said device within the frequency -band through which said backward wave device is tunable, voltage generating means adapted to render said backward wave device oscillatory to produce an electromagnetic energy output signal at the frequency of each injected locking signal, means for shifting said backward wave device from said oscillatory mode to said amplifying mode, means for receiving transmitted output signals reflected from said surface including a mixer cooperating with said locking signals and an intermediate frequency amplifier, a circuit interconnecting said receiving means and said locking signal means including local oscillator means connected to beat with each of said locking signals from said injection oscillator means to provide sidebands above and below each injection frequency, filtering means for passing only the upper or lower portions of said sidebands, and means `for applying the non-suppressed portion of said sidebands to said mixer input of said receiving means, whereby incoming reflected signals at each of said plurality of transmitted frequencies are progressively mixed with the corresponding sideband signal from said filter means.

2. An electrical system which is adapted to be selectively conditioned for transmitting and receiving a plurality of signals over a plurality of discrete `frequencies comprising a backward wave voltage tunable `device tunable through a predetermined frequency range, oscillator means connected to inject a plurality of locking signals into said backward wave device within the frequency range through which said backward wave device is tunable, voltage generating means adapted to render said device ioscillatory to provide power output signals at the frequency of each of said locking signals, means heating with said locking signals to provide a set of sidebland signals above and below the frequency of said oscillator means, means for receiving said output signals returning at the frequency of said locking signals, said receiving means including means for mixing the sideband signals with the returning output signals.

3. An electrical tuning system for alternately transmitting and receiving electromagnetic energy at a plurality of discrete frequencies comprising a backward wave voltage tunable device tunable through a predetermined frequency range, injection oscillator means for injecting a plurality of locking signals into said backward wave device within the frequency band through which said backwand wave device is tunable, voltage generating means for applying a voltage to said tunable device at a level adapted i4 to tune said device to a frequency corresponding to the frequency of each of said locking signals, thereby transmitting an electromagnetic energy output signal at the frequency of each of said locking signals, means for receiving said transmitted signals including a receiving antenna, a first mixer connected to said antenna, an intermediate .frequency amplifier coupled to said first mixer, an intermediate frequency oscillator and a second mixer fed by said intermediate frequency oscillator, said second mixer beating the output of said -intermediate frequency oscillator with said signals from said injection oscillator means to provide a set of sidebands above and below each injection frequency, means for filtering out one of said sets of sidebands, said first mixing means beating the nonsuppressed set of sidebands against said transmitted signals, and means fed by said intermediate frequency amplifier for detecting said intermediate frequency signals,n

4. A radar system for transmitting and receiving a plurality of signals at selected frequencies comprising a backward wave voltage tunable device tunable through a predetermined ifrequency range, oscillator means for injecting a plurality of locking signals into said backward wave device within the frequency range through which said backward wave device is tunable, voltage generating means adapted to render said device oscillatory to provide power `output at the frequency of each of said locking signals, means beating with said locking signals to provide a set of sideband signals above and below the frequency of said oscillator means, means for receiving said oscillatory power output of said backward wave device reflected at the frequency of said locking signals, said receiving means including means for mixing one set of sideband signals with the reflected power output signals at the frequency of the corresponding locking frequency of said backward wave device.

5. In combination, a backward wave voltage tunable traveling wave tube having an oscillatory and an amplifying mode of operation, said tube including a wave interaction path having a refiectionless termination at each end of said path, means for forming an electron beam which flows along said path, oscillator means for applying a plurality of driving locking signals at discrete frequencies to said wave interaction path, a first voltage regulating means for applying la plurality of predetermined voltage levels to said backward wave device to voltage tune said device to the individual frequency corresponding to said locking signals, a second voltage regulating means cooperating with said traveling wave tube after application of each locking signal thereby rendering said backward wave device oscillatory at the frequency corresponding to said predetermined voltage levels, means for extracting said oscillations from said interaction path, means for transmitting said extracted oscillations in the form of signals, and mixing means cooperating with said locking signals to receive said transmitted signals at predetermined frequencies.

6. A radar system for transmitting and receiving a plurality of signals at selected frequencies comprising a backward wave voltage tunable device tunable through a predetermined frequency range, oscillator means for injecting a plurality of pulsed locking signals into said device, means for sequentially voltage tuning said device through each #of said locking signals, voltage generating means to initiate oscillation in said device at the frequency of each locking signal to which said device is tuned, a first mixer and intermediate frequency amplifier, means for injecting said locking signals into said first mixer, oscillator means beating with said locking signals to provide sidebands above and below the frequency of each locking signal, a second mixer tuned to the frequency of said intermediate frequency amplifier and adapted to receive signals transmitted by said backward wave device, and means cooperating with said sideband signals to provide a selected sideband frequency to said first mixer at the time of reception of the corresponding transmitted signal.

7. A radar system for transmitting electromagnetic energy comprising a backward wave voltage tunable dev-ice tunable through a predetermined frequency range, oscillator means for injecting a plurality of locking signals into said device at individual frequencies within said frequency range, means fior voltage Ituning said' backward wave device to a first of said locking signals, voltage ,Generating means connected to activate said backward wave device at a first voltage level to produce an electromagnetic energy signal at substantially the frequency of said first locking signal, means for inactivatlng said backward wave device, means for receiving said transmitted energy in the form lof a return `echo means for tuning said backward wave device to a second of said locking signals, and means for activating said backward wave device at a second voltage level to produce energy at substantially the same frequency as said second locking signal.

8. An electrical system for sequentially transmitting and receiving a plurality of signals at selected frequencies comprising a backward wave voltage tunable device tunable through a predetermined frequency range, a traveling wave amplifier feeding said backward wave device, a plurality of locking signals feeding said traveling wave amplifier, a regulatory circuit for progressively tuning said backward wave device through each of said locking signals,s,aid regulatory circuit adapted to cause said backward wave device to transmit at the frequency of the lock lli regulatory circuit adapted to provide a plurality of output voltages to cause said backward wave device to transmit at the frequency of said locking signal to `which said device is tuned, a receiver for receiving said transmitted signals in the form of echoes including a first mixer fed by said transmitted signals and feeding an intermediate frequency amplifier, a second mixer fed by said locking signals, an oscillator feeding said second mixer and tuned to the frequency of said intermediate frequency amplifier, said oscillator beating with said locking signals to provide sidebands above and below the frequency of each of said locking signals, a filter connected between said first and second mixer to pass only one of said sideband frequencies to said first mixer at the time of reception of the corresponding transmitted signal, and timing circuitry progressively controlling the voltage applied to said device t0 terminate oscillation at the frequency of each of said locking signals, said filter including adjustable frequency peaking means tunable to a plurality of frequencies.

10. A radar system for transmitting and receiving a plurality of signals at selected frequencies comprising a backward wave voltage tunable device tunable through a predetermined frequency range, said device including a wave interaction path having a reiiectionless termination at each end of said path, means for forming lan electron beam which flows along said path, means for injecting a ing signal to which said device is tuned, a receiver for receiving said transmitted signals in the `form of echoes including a first mixer and intermediate"frequency amplifier connected thereto, a second mixer fed by said locking signals, an oscillator feeding said second mixer and tuned to the frequency of said intermediate frequency amplifier, said oscillator beating with said locking signals to provide sidebands labove `and below the frequency of each of said locking signals, a filter connected between said first and second mixer to pass only one of said sideband frequencies to said first mixer at the time of reception of the corresponding transmitted signal, and timing circuitry sequentially controlling the termination of oscillation in said backward wave device at a frequency corresponding to the frequency of each of said locking signals.

9. A radar system for transmitting and receiving a plurality of signals `at selected frequencies comprising a backward wave voltage tunable device tunable through a predetermined frequency range, a traveling wave amplifier feeding said backward wave device, a plurality of locking signals feeding said traveling wave amplifier, a regulatory circuit for progressively tuning said backward wave device through each of said locking signals, said plurality of locking signals into said device at a plurality of discrete frequencies, a first mixer fed by said locking signals, an intermediate frequency oscillator feeding said first mixer to produce sidebands above and below the frequency of said locking signals, a second mixer, a filter adapted to pass a single sideband connected between said first and second mixer, antenna means for introducing transmitted return signals into said second mixer, an intermediate frequency -amplifier fed by said second mixer, whereby said mixed signals are amplified, `and voltage regulating means for sequentially voltage tuning said device to the frequency of each of said locking signals.

References Cited in the le of this patent UNITED STATES PATENTS 2,408,791 Magnuski Oct. 8, 1946 2,447,392 Byrne Aug. 17, 1948 2,654,832 Robinson Oct. 6, 1953 2,706,251 Russell et al Apr. 12, 1955 2,748,268 Whinnery May 29, 1956 2,760,161 Cutler Aug. 21, 1956 2,827,627 Arams Mar. 18, 1958 2,862,203 Skaraeus et al Nov. 25, 1958 

1. A RADAR SYSTEM FOR ALTERNATELY TRANSMITTING ELECTROMAGNETIC ENERGY TO A REFLECTING SURFACE AND RECEIVING ELECTROMAGNETIC ENERGY FROM SAID SURFACE COMPRISING A BACKWARD WAVE VOLTAGE TUNABLE DEVICE HAVING AN OSCILLATORY MODE AND AN AMPLIFYING MODE OF OPERATION AND TUNABLE THROUGH A PREDETERMINED FREQUENCY RANGE, MEANS FOR CYCLICALLY TUNING SAID DEVICE THROUGH SAID FREQUENCY RANGE, INJECTION OSCILLATOR MEANS FOR INJECTING A PLURALITY OF LOCKING SIGNALS INTO SAID DEVICE WITHIN THE FREQUENCY BAND THROUGH WHICH SAID BACKWARD WAVE DEVICE IS TUNABLE, VOLTAGE GENERATING MEANS ADAPTED TO RENDER SAID BACKWARD WAVE DEVICE OSCILLATORY TO PRODUCE AN ELCTROMAGNETIC ENERGY OUTPUT SIGNAL AT THE FREQUENCY OF EACH INJECTED LOCKING SIGNAL, MEANS FOR SHIFTING SAID BACKWARD WAVE DEVICE FROM SAID OSCILLATORY MODE TO SAID AMPLIFYING MODE, MEANS FOR RECEIVING TRANSMITTED OUTPUT SIGNALS REFLECTED FROM SAID SURFACE INCLUDING A MIXER COOPERATING WITH SAID LOCKING SIGNAL AND AN INTERMEDIATE FREQUENCY AMPLIFER, A CIRCUIT INTERCONNECTING SAID RECEIVING MEANS AND SAID LOCKING SIGNAL MEANS INCLUDING LOCAL OSCILLATOR MEANS CONNECTED TO BEAT WITH EACH OF SAID LOCKING SIGNAL FROM SAID INJECTION OSCILLATOR MEANS TO PROVIDE SIDEBANDS ABOVE AND BELOW EACH INJECTION FREQUENCY, FILTERING MEANS FOR PASSING ONLY THE UPPER OR LOWER PORTIONS OF SAID SDEBANDS, AND MEANS FOR APPLYING THE NON-SUPPRESSED PORTION OF SAID SIDEBANDS TO SAID MIXER INPUT OF SAID RECEIVING MEANS, WHEREBY INCOMING REFLECTED SIGNALS AT EACH OF SAID PLURALITY OF TRANSMITTED FREQUENCIES ARE PROGRESSIVELY MIXED WITH THE CORRESPONDING SIDEBAND SIGNAL FROM SAID FILTER MEANS. 